1. Field of the Invention
The present invention relates to doppler shift correction systems for use with high speed vehicles.
2. Description of the Prior Art
Radio communication involving high speed, high-performance aircraft, for example, may result in large doppler frequency shifts from the nominal carrier frequencies of the transmitted and received signals. Typically, the speeds of such aircraft are as high as 2,000 mph. Doppler shifts at such speeds are in the order of 1,200 Hz from the nominal carrier frequency at carrier frequencies on the order of 400 MHz. At higher carrier frequencies, the doppler shift may be even larger. Where the high performance aircraft requires doppler tracking for reception and communicates with a station which does not employ doppler tracking, the high performance aircraft must alter its transmission to compensate for doppler shift, making the net frequency shift approach zero at the station without tracking.
For example, where the remote station is simultaneously communicating with a number of high speed aircraft, and communications with the individual aircraft involve different doppler shifts, the remote station generally cannot track all of the different doppler shifts simultaneously. Thus, communications from the individual high speed aircraft to the remote station must be pre-compensated for doppler shift.
In some applications, a station moving at a low speed or a fixed station communicates mainly with other low speed or fixed stations and numerous high speed stations and hence for reasons of economy does not employ doppler tracking techniques. Thus, the high performance aircraft or station again must pre-compensate its transmissions to the fixed or low speed station for doppler shift, so that the net frequency shift in the signals received at the receiving station approaches zero. Some prior art systems require that a system master oscillator output frequency be modified to compensate for doppler shift. However, it is undesirable to change the master oscillator output frequency because it is generally the station reference.
Prior art systems, such as those described in U.S. Pat. Nos. 3,317,909 and 3,325,736 issued to R. M. Waetjen on May 2, 1967 and June 13, 1967, respectively, accomplish a doppler compensation, in effect, by frequency multiplying, inverting and retransmitting the signal received from the remote station. More specifically, such prior art systems first subtract a reference signal at the nominal reception frequency from the received signal. The difference signal is then frequency multiplied by the ratio of the nominal transmission frequency to the nominal reception freqency. The resultant signal is subtracted from a reference signal at nominal transmission frequency and the difference signal is transmitted to the remote station. The reference signals for the nominal transmission and reception frequencies are both generated from a submultiple of the nominal reception frequency in a synthesizer. Such systems are disadvantageous in that they require the use of tuned radio frequency components, as opposed to lighter, more dependable, less expensive direct current components. Another disadvantage of such systems is that the relationship between transmission and reception frequencies is limited to ratios that may be embodied in a practicable frequency multiplier. Moreover, in such systems the response of a station to a received signal is fixed at a single ratio, thus making the system unsuitable for use in situations where the nominal transmission and reception frequencies are independently variable.
The present invention provides digital means for generating a doppler corrected transmission, suitable for use in systems where the nominal transmission and reception frequencies are different and are frequently changed over a wide band of frequencies. In addition, the present invention provides doppler compensation means suitable for situations where the nominal transmission and reception frequencies are independently variable. The present invention is also suitable for use in frequency shift keying systems, wherein the frequency transmitted varies in accordance with binary data to be transmitted.